Objective Discuss Expansion Valves and Refrigerants Heat Exchangers Learn about different types Define Heat Exchanger Effectiveness (ε)

AEV Maintains constant evaporator pressure by increasing flow as load decreases

Thermostatic Expansion Valve (TXV) Variable refrigerant flow to maintain desired superheat

Refrigerants

What are desirable properties of refrigerants? Pressure and boiling point Critical temperature Latent heat of vaporization Heat transfer properties Viscosity Stability

In Addition…. Toxicity Flammability Ozone-depletion Greenhouse potential Cost Leak detection Oil solubility Water solubility

Refrigerants What does R-12 mean? ASHRAE classifications From right to left ← # fluorine atoms # hydrogen atoms +1 # C atoms – 1 (omit if zero) # C=C double bonds (omit if zero) B at end means bromine instead of chlorine a or b at end means different isomer

Air-liquid Tube heat exchanger Plate heat exchanger Heat exchangers Air-air

Some Heat Exchanger Facts All of the energy that leaves the hot fluid enters the cold fluid If a heat exchanger surface is not below the dew point of the air, you will not get any dehumidification Water takes time to drain off of the coil Heat exchanger effectivness varies greatly

Example: What is the saving with the residential heat recovery system? Furnace 72ºF 32ºF 72ºF Outdoor Air For ε=0.5 and if mass flow rate for outdoor and exhaust air are the same 50% of heating energy for ventilation is recovered! For ε=1 → free ventilation! (or maybe not) 52ºF Exhaust Gas Combustion products Fresh Air

Heat Exchanger Effectivness (ε) C=mc p Location BLocation A T Hout T Cin T Cout T Hin Mass flow rateSpecific capacity of fluid

Air-Liquid Heat Exchangers Fins added to refrigerant tubes Important parameters for heat exchange? Coil Extended Surfaces Compact Heat Exchangers

What about compact heat exchangers? Geometry is very complex Assume flat circular-plate fin

Overall Heat Transfer Q = U 0 A 0 Δt m Overall Heat Transfer Coefficient Mean temperature difference

Heat Exchangers Parallel flow Counterflow Crossflow Ref: Incropera & Dewitt (2002)

Heat Exchanger Analysis - Δt m

Counterflow For parallel flow is the same or

Counterflow Heat Exchangers Important parameters:

What about crossflow heat exchangers? Δt m = F·Δt m,cf Correction factor Δt for counterflow Derivation of F is in the book: ………

Example: Calculate Δt m for the residential heat recovery system if : mc p,hot = 0.8· mc p,cold t h,i =72 ºF, t c,i =32 ºF For ε = 0.5 → t h,o =52 ºF, t h,i =48 ºF → R=1.25, P=0.4 → F=0.89 Δ t m,cf =(20-16)/ln(20/16)=17.9 ºF, Δ t m =17.9 ·0.89=15.9 ºF

Overall Heat Transfer Q = U 0 A 0 Δt m Need to find this

Heat Transfer t P,o From the pipe and fins we will find t F,m t

Resistance model Q = U 0 A 0 Δt m Often neglect conduction through tube walls Often add fouling coefficients

Heat exchanger performance (Book section 11.3) NTU – absolute sizing (# of transfer units) ε – relative sizing (effectiveness) Criteria NTU εPRP crcr

Fin Efficiency Assume entire fin is at fin base temperature Maximum possible heat transfer Perfect fin Efficiency is ratio of actual heat transfer to perfect case Non-dimensional parameter

Summary Calculate efficiency of extended surface Add thermal resistances in series If you know temperatures Calculate R and P to get F, ε, NTU Might be iterative If you know ε, NTU Calculate R,P and get F, temps